The invention relates to screen-printed metal oxide varistor electrodes. More specifically, the invention relates to the fabrication of varistors with solderable, largely non-noble metal electrodes.
A zinc oxide (ZnO) varistor device comprises, typically, a disk of varistor material having electrodes affixed to at least one of the major surfaces thereof. Although there are a number of methods for attaching electrodes to the varistor material, the conductive leads for connecting the varistor to an electrical circuit are usually attached to the electrodes by soldering. Solderable electrodes for varistors used in commercial applications are either screen-printed silver or flame-sprayed brass. Other processes for attaching electrodes include evaporation and electroless metal deposition, for example. THese methods are technically feasible, but except for the larger, higher cost varistor devices are uneconomical due, in part, to the fact that they are not easy to automate.
A method for attaching varistor electrodes which is economical and easily automatable is screen printing. In fact, ZnO varistors for electronic equipment protection are currently fabricated using screen-printed silver electrodes. A disadvantage associated with all-silver or all-noble metal electrodes is that silver is expensive. Therefore, it is desirable to find a replacement for silver. Air fireable, screen-printable, base metal conductive pastes of such metals as nickel (Ni), aluminum (Al), and chromium (Cr) are available and in principle are usable for varistor electrodes. These materials, however, are not easily solderable and have a high resistivity compared to that of silver-based materials. For example, nickel, aluminum, and chromium electrodes have resistivities of 40-80, 20-50, and 500-900 milliohms/square, respectively. Silver-based electrodes with resistivities of 2-4 milliohms/square are common.
The present invention provides a metal oxide varistor with solderable, low cost, largely non-noble metal electrodes.